This invention relates to an improved valve pin for a valve gating injection molding system.
In the past, a wide variety of valve gating systems have been used with varying success for injection molding different materials in different applications. However, while having a variety of different arrangements for providing heat in the gate area, these previous systems have primarily emphasized the valving system and failed to appreciate the degree to which heat transfer to the gate area is critical to the operation of the whole system. While these previous systems do operate for some materials under favourable conditions, their performance is not optimum and their performance is particularly unsatisfactory for more difficult materials under difficult conditions. Without sufficient heat transfer to the gate area, increased injection pressures are required as well as increased force on the valve pin which both in turn lead to subsequent operating difficulties and increased cost. In addition, the heaters in many of these previous systems are subject to the problems of burn out and overheating.
More recently, it has become highly desirable to mold new high density materials such as up to 60% glass-filled nylon to replace aluminum molded products. Conventional valve gating systems which do not provide sufficient uniform heat to the gate area have been unable to mold these types of materials. With conventional thermoplastic materials which gradually soften with increased temperatures, closing problems have been overcome by increasing the valve pin force to the area of 400 to 800 pounds. With crystalline materials even this short sighted solution is not available because the melt solidifies very sharply with reduced temperatures. By providing sufficient heat to the gate area the present invention will enable most applications to be run with reduced valve pin forces in the area of from 150 to 300 pounds.
Furthermore, in some molding applications it is desirable to form a hole in the molded product to coincide with the gate opening. This may be done by providing for the valve pin to penetrate through the plastic part and seat against the core of the mold as well as in the gate itself. This requires an even transfer of sufficient heat from the upper portion of the heater cast and the melt to the tip of the valve pin where in the past it has been very difficult to maintain and particularly to control temperatures at the necessary level.